A slot array antenna comprises a plurality of slots formed within a plate portion of the rectangular waveguide. FIGS. 21a and 21b show wave propagation modes within the rectangular waveguide. The wave propagation mode, within the rectangular waveguide is a dominant mode (TE.sub.10 or TE.sub.01 wave) the attenuation of which is the smallest, and which may be represented by means of orthogonal coordinates. If the cutoff frequency is designated fc, the speed of light as c, and the length of the long side of the waveguide as a, the waveguide is used within the frequency range between fc=c/2a and fc(20)=c/a at which attenuation of another higher order mode occurs. Accordingly, the long side length a of the waveguide is between EQU a=.lambda./(1.06.about.1.3.about.1.56)
where .lambda. is the free space wavelength.
The slots of a conventional slot array antenna as shown in FIG. 22 are formed within a plate portion of the above described waveguide. As shown in FIG. 22, the direction of the current is inverted at every one-half wavelength interval .lambda.g/2(.lambda.g is the wavelength within the waveguide) and the direction of the inclination of each slot is accordingly oppositely disposed with respect to each adjacent slot. Thus, all of the Z-components of the resultant electrical field of the wave radiated from each slot are oriented in one direction, and the Y-components are disposed in opposite phase with respect to each other so as to offset or cancel each other. As a result, a wave having linear polarization is radiated from the slots. The width of the beam in the x-y plane is between 16.degree. and 20.degree. and that in the x-z plane is between 1.degree. and 2.degree. which is in proportion to the number of slots and hence is narrow.
Since the beam width in the horizontal plane x-z is narrow and the beam width in the vertical plane x-z is wide, the gain of the above described slot array antenna is small. Consequently, the antenna is improper to use as an antenna for the communication, broadcasting and similar fields, although it is useful within radar systems.